The principle of force rebalancing is widely used in the field of instrumentation. For example, it is used in instruments measuring force or related quantities such as accelerometers. In the accelerometer, the suspended seismic mass is measured by an optical, mechanical, or electrical position sensor. The function of the position sensor is to detect any deviation of the mass from a reference point or null position and to provide signals to a drive means to exert a counter force on the mass to drive it back to the null position.
In U.S. Pat. No. 3,877,313, assigned to the same assignee as the present invention there is disclosed an accelerometer in which the displacement and restoration to null position of the seismic mass or pendulum is accomplished by the use of separately energized capacitive torquer plates for restoration to null and by use of a pair of pickoff plates to detect the displacement angle. Specifically, in the teaching of the above-identified patent, a pair of pickoff electrodes separate from the pair of torquer electrodes are required. Additionally, a source of sinusoidal excitation phase-locked at the sampling rate is required to be applied to an electrically separated portion of the pendulum. Moreover, pickoff amplifiers and a differential amplifier are required to process the pickoff signal.
One disadvantage of the described device is that separate pickoff electrodes are required which complicates the plating of the electrodes and requires two additional electrical connections to the accelerometer. Also, the area for the pickoff electrodes substracts from the total available area limiting the area available for the torquer electrodes.
Another disadvantage of the prior art is that the pendulum had to be a composite structure of non-conducting ceramic, plated in two distinct areas responsive to the torquer electrodes and pickoff excitation respectively. Since the hinged or "necked down" portion of the prior art pendulum is metallic, the composite structure requires two electrical connections to be made external to the accelerometer; one to each of the two plated areas.
A third disadvantage of the prior art is that a source of sinusoidal excitation is required to be applied to part of the pendulum opposite the pickoff electrodes. Additional electronics are required to generate the sinusoid. Moreover, this excitation, when applied to the pendulum, may produce undesired forces on the pendulum detracting from accelerometer performance.